The present invention relates to an improved method of joining semiconductor substrates used in a semiconductor device.
In the manufacturing of a semiconductor device, the need often arises to join one semiconductor substrate with another. For example, in order to prepare a high breakdown voltage integrated circuit, various types of dielectric-isolated substrates and their fabrication processes have been proposed to increase a breakdown voltage and improve element isolation. In the process for manufacturing such a dielectric-isolated substrate, the following techniques have been taken into consideration. A semiconductor substrate with an element formation region is supported by another material, or joined with a homogenous or heterogeneous substrate. With this kind of technique, it is necessary to fulfill structural and manufacturing conditions such as the following. First, any grooves on the joint surface of the substrate must be completely filled with a joining agent or the like. Second, harmful impurities cannot be allowed to diffuse into the element formation region of the substrate. Third, heat resistance must be provided. Finally, the process must allow the substrate to be free from crystal defects such as dislocations.
According to a first conventional method of manufacturing a semiconductor substrate, as described by U.S. Davidsohn and Faith Lee, Proceedings of IEEE, Vol. 57, No. 9, September, 1532 (1969) or in U.S. Pat. No. 4,393,573, polycrystalline silicon constituting a support is deposited on a V-grooved semiconductor substrate having a dielectric film thereon to fill the V-grooves. Subsequently, the opposite surface of the substrate having a joint surface is polished by a predetermined thickness to prepare a dielectric isolation structure. However, since deposition according to this method is performed to a thickness of more than 350 .mu.m by hydrogen reduction reaction using a gas such as trichlorosilane (SiHCl.sub.3) gas, a large amount of source gas must be used, and a long deposition time is necessary. In addition, in order to prevent the substrate from warping after the manufacturing process, conditions such as deposition rate and temperature must be strictly controlled. Furthermore, an SiO.sub.2 film must be formed by depositing SiO.sub.2 a few times to prevent the warping of the substrate, resulting in increased cost.
According to a second conventional method, as described in U.S. Pat. No. 3,909,332, a mixture of silane (SiH.sub.4), diborane (B.sub.2 H6), and oxygen (O.sub.2) is used as the source gas for the glass film to be deposited. After depositing the glass film on the joint surfaces of the two substrates by CVD, the substrates are placed together at the glass film portions and heat-treated under pressure. With this method, however, grooves and projections or recesses on the junction surface of either substrate cannot be filled completely because of the thinness of the glass film. In addition, it is necessary to apply pressure to the substrates. If the grooves and projections or recesses cannot sufficiently filled with the glass film, cracks may occur in the subsequent polishing process, or holes may be left. A resist used in the subsequent photolithography or the like is inserted in the holes and undesirably becomes a contamination source. Furthermore, since the pressure acts to join the substrates, crystal defects are formed in the element formation region of the semiconductor substrate due to the mechanical stress.
According to a third conventional method, as described in Japanese Patent Disclosure No. 53-57978, grooves and projections or recesses formed on the junction surfaces of the substrates are filled with a uniform mixture having a suitable composition of silicon dioxide powder and boric acid or boric anhydride as a joining agent. A support is placed on the substrate, and the resultant structure is heated and compressed, thereby joining the substrate and the support. In order to fill the grooves and the like without gaps, however, it is necessary to apply vibrations with a vibrator or ultrasonic waves, so that the fabrication process is complicated. In addition, it is impossible to obtain a uniform composition. The nonuniform composition varies the melting point of the joining agent. In the heat-treatment process subsequent to the isolation polishing, part of the element region is subjected to movement. Furthermore, since crystal defects are induced by mechanical stress from the compression upon adhesion, electrical characteristics are degraded.
According to a fourth conventional method, as described by M. Kimura, K. Egami, and M. Kanamori, Applied Physics Letters, Vol. 43, No. 3, August 263 (1983), a glass powder, having a melting point of less than 1,000.degree. C. and containing lead-borosilicate (Pb-B.sub.2 O.sub.3 -SiO.sub.2) or a heavy or alkali metal, is dispersed in an organic solvent. After applying the resulting substance to one substrate with a spinner or the like and heated, the substrate is placed on another substrate and reheated under pressure. Because of the low melting point of the glass powder used as a joining agent, however, substrates joined with this method cannot be used in high temperature regions, contamination of the substrates by the heavy metal atoms is significant, and a large number of voids are formed. Furthermore, the compression process upon adhesion is also necessary, thus presenting the same shortcomings as the third method.
None of the conventional methods, then, are without shortcomings, and a semiconductor substrate manufacturing method which fulfills all of the desired conditions is yet to be seen.